1. Field of the Invention
This invention relates to gas compressor unloaders, and in particular to an unloader for use in gas transmission and refrigeration compressors.
2. Description of the Prior Art
In the operation of gas compressors having compressor cylinders with compressor pistons reciprocating therein, it is frequently desirable to reduce the volumetric output of the compressor. Variable speed motors may be used for adjusting the output of such reciprocating-type compressors, however, the gas pressures, torque and work done per revolution remain constant when only the engine speed is varied. Adjusting pressure cylinder capacity by unloading means is therefore preferable because torque, capacity, and power consumption per revolution may also be reduced thereby. Also, unloader devices which vary compressor cylinder capacity are adaptable for use with synchronous motors, thereby avoiding the additional expense and complexity of variable speed motors.
Such compressor cylinder capacity may be controlled by a variety of different devices. By way of example, a compressor cylinder end may be deactivated by physically lifting the suction valves to an unloaded position with a mechanical linkage system extending through the valve cap cover or by completely removing the suction valves. However, valve assemblies are difficult to move when subjected to the relatively high gas pressures within the gas compressor and the required mechanical linkage tends to cause difficulty in maintaining a gas-tight seal. Also, removing a suction valve assembly is generally not possible with the compressor in operation.
Another prior art method of unloading compressor cylinders is characterized by the use of mechanical fingers which extend through air passages in a valve seat for selectively holding the valve members in their open positions whereby virtually infinite clearance is provided and no compression occurs. However, the fingers are susceptible to breakage and occupy space within the gas passages thereby increasing flow resistance and gas temperatures.
Two other methods of unloading compressor cylinders are exemplified in our U.S. Letters Patent No. 4,043,710 wherein a clearance bottle is disclosed selectively communicating with a compressor cylinder and an unloader piston is provided for selectively communicating the compressor cylinder with suction passages. However, clearance bottles are limited by their volume in the amount of unloading capacity they can provide and such unloader pistons are limited in their application to valve assemblies having central passages therethrough.
In refrigeration gas compressors, it is generally desirable for an unloader mechanism to control compressor cylinder capacity in response to an operating condition of the system, such as temperature or gas pressure. Although it is known to automatically unload a reciprocating compressor in response to an operating condition of a refrigeration system, a relatively powerful actuating mechanism was typically required to overcome the relatively high differential gas pressures within the compressor and thereby move an unloader mechanism to its unloaded position. See, for example, the Moody Pat. No. 1,985,642 and the Cooper Pat. No. 2,726,032.
Prior art compressor unloaders, however, tended to be either complex in structure and manufacture, susceptible to breakage and other maintenance problems, and detrimental to the efficiency of a compressor.